Physic: I don't know how to do (b), (c), would you mind to teach me, thanks?

(a) the horizontal component of the normal force exerted by the track on the marble

(b) tanΘ = F / mg → the horizontal and vertical components
F = m*g*tanΘ = 0.020kg * 9.8m/s² * tan20º = 0.071 N

(c) F = 0.071 N = mv²/r = 0.020kg * v² / 0.10m
so v = 0.60 m/s or 60 cm/s
and period T = 2πr / v = 2π*10cm / 60cm/s = 1.05 s

a) centripetal force is provided by the centripetal acceleration ca = V^2/r

b) centripetal force CF = m*ca = m*V^2/r = m*r*g*tan 20/r = 0.02*9.8*0.364 = 0.07 N

c) speed V = √r*g*tan 20 = √0.364*9.806*0.1 = 0.60 m/sec
c') period T = C/V = 6.2832*0.1/0.6 = 1.05 sec